Recent advances in semiconductor manufacturing technology have resulted in the increased miniaturization of integrated semiconductor components, generally known as "chips". The new chips are able to perform more complex functions, and at a faster rate, than their predecessors, yet are often the same size, or smaller. In order to perform these complex functions, the new chips consume more power than their predecessors and, as a consequence, generate more heat. This is significant because most chips should be operated below 100.degree. C. If a chip becomes too hot, the semiconductor junctions, which constitute the basic electrical elements within the chip, have a tendency to break down and the chip may malfunction., Thus, it is necessary to efficiently extract the heat, or otherwise cool the chips, while they are operating to insure that they continue to function properly.
Thermal packages have been provided for chips which, in addition to providing chips with protection, include a means for extracting the heat generated by the chips. Inside the chamber is some type of heat transfer device, such as pistons or other members formed of thermally conductive material, that are each in contact with a seperate chip and are all in contact with an external heat sink. The external heat sink usually comprises either a set of cooling fins that are integral with the outer surface of the package, or a cold plate with liquid circulating therethrough. The heat generated by the chips is extracted by the heat sink through the heat transfer devices.
In order for heat to readily flow from the chip through the heat transfer device, there should be a minimal amount of thermal resistance between the chip and the heat transfer device Consequently, the heat transfer device must be in physical contact with the chip. Many thermal packages are provided with heat transfer devices that are biased against the adjacent chips by springs or other mechanical devices. These devices thus exert a stress-inducing, force on the chip. This stress may be aggravated due to the repetitive thermal expansion and contraction of the heat transfer device as the quantity of heat passing through it varies, causing the heat transfer devices to expand and contract at a rates different than those of the chips they are in contact with. Moreover, the heat transfer devices may further stress the chips they are in contact with by imparting external strains and transmitting mechanical vibrations. These stresses may occur both in during the normal operation of the device or as a consequence of movement of the device such as in shipping or instillation. Over time, the changes in mechanical stress may fatigue the chips so that they are torn loose from the circuit they are attached to, break, or are otherwise rendered useless.
Furthermore, the heat transfer devices should be in good physical contact with the external heat sink in order to maintain good thermal conductivity therebetween. To provide the necessary contact, many thermal packages are integral assemblies wherein the chip, the heat transfer device and the external heat sink cannot be readily disassembled from each other. This type of assembly makes it difficult to perform maintenance on just one part of the package. For instance, if a chip in the package malfunctions, it may be difficult, to gain access to it so it may be replaced, and so the entire package may have to be replaced. This situation would add to the overall cost of operating a device that contains this type of thermal package.
Moreover, many new chips need to be supplied with a relatively high drain voltage level in order to insure their efficient operation. Normally this voltage is supplied by conductors on the printed circuit board to which the chip is attached. In a thermal package, the chip may be attached to a small-sized printed circuit board within the package that may not have sufficient space to accommodate the conductors necessary to supply the drain voltage.
Another consideration in the packaging of semiconductor chips is that frequently the chips attached to a particular circuit generating the same amount heat are subjected to differing cooling conditions. This may be because of their locations in the circuit that they are part of. For instance, a chip closer to a cooling source, such as a fan or an opening, may operate at lower temperature than a chip further away. When this occurs, signal transmission between chips may be degraded because of differing voltage levels and noise margins on the chips as a result of their being operated at substantially different temperatures.